The present invention concerns a ceramic catalytic membrane reactor for the separation of hydrogen and/or its isotopes from fluid feeds. More particularly, this invention relates to a tubular membrane reactor, wherein said membrane is selective for hydrogen and its isotopes, and acts further as a catalyst, thus allowing through it an oxidative diffusion of hydrogen. Water is recovered downstream from the membrane.
Separation of hydrogen from gas feeds is quite a common problem in conventional industrial processes, e.g., in the processes of dehydrogenation of organic compounds, in molecular reforming of hydrocarbons, and, more generally, in all those processes where a controlled atmosphere is involved. Normally used equipment includes fixed bed type catalytic reactors, cryogenic systems or systems based on polymeric membranes.
For example, integrated dehydrogenation/separation processes use fixed bed type catalytic reactors, wherein the catalyst is confined within a Pd/Ag membrane. The latter, due to its selectivity, allows only hydrogen to flow outward through the membrane. Thus, hydrogen is removed from the reaction area and the concerned thermodynamic equilibrium is shifted towards the right side. Similarly, N. Itoh, in AIChe J., 33, N.9, 1576 (1987), described the use of a fixed bed membrane reactor comprising a proper catalyst to dehydrogenate cyclohexane to benzene, wherein said membrane was a 200 um thick palladium tube.
Other processes using membranes that were recently developed involve depositing a thin selective film (e.g., Pd or Pd/Ag) on microporous ceramic or porous glass substrates.
In the ceramic material field, the work of Iwahara et al., in Sol. State Ion. 18-19, 1003 (1986) is to be mentioned, wherein a special ceramic material is described as a proton conductor to be used as solid electrolyte to extract electrochemically hydrogen from gas mixtures, as well as, for example, in water vapor electrolysis.
All the membrane processes referred to above exploit the membrane selectivity properties only, and strictly depend upon the ratio of the partial pressures of the permeating gas on the opposite sides of the membrane.
Some solutions have been recently suggested which would allow one to overcome the above problem by combining the selective permeability properties of the membranes at issue with a catalytic activity; that is, the hydrogen, passing through a selective membrane with proper catalytic activity and coming into contact on the other side of the membrane with an oxygen-containing gas, oxidizes and forms water, so that no partial pressure of the permeating gas exists downstream from the membrane.
Itoh (J. Chem. Eng. of Jap., 23, n.1, 81, (1990) applied this principle to an integrated process for cyclohexane dehydrogenation (on one side of the membrane) and hydrogen oxidation (on the opposite of the membrane), using a palladium membrane, while Buxbaum & Hsu (J. Nuc. Mat., 141-143, 238 (1986) applied the sample principle to extracting tritium from a liquid breeder of a nuclear fusion reactor.
With reference to the nuclear field, a main problem of the cycle of the use of nuclear fuel in a fusion reactor is to extract the tritium from the reactor blanket. For example, an actual case involves extracting tritium from the ceramic material into which this isotope is formed by washing the ceramic material with a hydrogen-containing inert gas flow (He or Ar). Both isotopes must then be removed from the inert carrier gas. Usually, such flow is fed to a reactor containing a catalyst bed and is oxidized with 0.sub.2. The resulting water is then separated by means of a cryo-condensation procedure or by a molecular sieve system. However, the gas resulting from the separation unit cannot be directly recycled, as it contains oxygen.
In such cases the ability to use a catalytic membrane reactor wherein the inert, isotope-containing gas flow is physically kept apart from the oxygen-containing flow, would be very advantageous.
Apart from the case of catalytic membranes entirely made of Pd or Pd/Ag, that cannot be used for industrial scale processes, the solution suggested by the prior art (Buxbaum et al., see above) involves the use of membranes obtained by depositing thin films of a catalyst material (such as, e.g., palladium) on a metal substrate (such as, for example, zirconium, vanadium, niobium, etc.).
However, such prior art solutions have a number of drawbacks, among which, first of all, are those due to the poor permeability of the metal substrate. In view of that, a large exchange surface area is required, and this, obviously, means very high costs both for the equipment and for the material for the catalyst film production.
Secondly, the suggested metal membranes are subject, under the desired working conditions, to embrittling and poisoning effects that could restrict their useful life.
It is further to be taken into account that, in order to have hydrogen and its isotopes react when permeating through the membrane, the membrane at least should reach a temperature ranging from 250.degree. C. to 450.degree. C. Due to the high thermal conductivity of the metal substrate, such temperatures unavoidably cause hydrogen leaks from the equipment seals.
Therefore, an object of the present invention is to provide a catalytic membrane reactor which overcomes the above problems, thus affording the separation and catalytic oxidation of hydrogen and/or isotopes thereof with a high efficiency and with a reduced exchange surface.